Slip-controlled hydraulic vehicle braking systems are known; reference is made, for example, to the published patent application DE 198 25 114 A1. The known vehicle braking system has a muscle-power actuable brake master cylinder to which, for example, two brake circuits are connected via one isolation valve per brake circuit. One or more wheel brake cylinders of the hydraulic wheel brakes are connected to each brake circuit, each wheel brake cylinder having a brake pressure build-up valve by which it is connected to the brake circuit for wheel-individual control of the wheel brake pressure. In addition, each wheel brake cylinder has a brake pressure reducing valve by which it is connected to a suction side of a piston pump for wheel-individual control of the wheel brake pressure, the vehicle braking system having in each brake circuit a piston pump to which all the wheel brake cylinders of this brake circuit are connected via their brake pressure reducing valves. A pressure side of the piston pump is connected to the brake circuit between the isolation valve and the brake pressure build-up valves.
In order to reduce the wheel brake pressure in one or more wheel brake cylinders during a slip control operation, the respective brake pressure build-up valve is closed and the brake pressure reducing valve is opened, so that brake fluid flows out of the wheel brake cylinder. A build-up or increase of the wheel brake pressure is effected with the piston pump through the open brake pressure build-up valve, the brake pressure reducing valve being closed during the pressure build-up or pressure increase. Brake fluid can be delivered back to the brake master cylinder through the isolation valve.